A magnetic resonance imaging (abbreviated to “MRI” hereinafter) system measures a density distribution, a relaxation time distribution, etc. of nuclear spins (referred to simply as “spins” hereinafter) in a desired examination area of an object to be examined by utilizing a nuclear magnetic resonance (abbreviated to “NMR” hereinafter) phenomenon, and then displays an image of the object in an arbitrary section based on the measured data.
Some of MRI systems have the image-taking function called MR angiography (abbreviated to “MRA” hereinafter) adapted for imaging blood flows. The MRA image-taking function is practiced by two methods, i.e., one using no contrast agent and other using a contrast agent. Generally, the method using the contrast agent is more superior in capability of imaging blood vessels and is able to obtain a blood vessel image with higher quality.
A typical example of the method using the contrast agent is combined one of a T1-shortening contrast agent, such as Gd-DTPA, and a gradient echo sequence with a short TR (Repetition Time).
That combined method is intended to image the vascular lumen filled with the blood containing the contrast agent at a higher contrast than other tissues based on the fact that, because spins in a blood flow containing the T1-shortening contrast agent have shorter T1 than surrounding tissues, those spins are less apt to saturate even at the same TR and hence generate higher signals than the other tissues in the relative sense.
Blood vessels can be imaged by measuring data (three-dimensional in practice) of a volume including blood vessels for a short time during which the contrast agent remains in the blood vessels, and by executing a projection process, e.g., a maximum intensity projection process, while the measured three-dimensional images are superimposed with each other. Therefore, an image-taking sequence used in MRA is generally on the basis of a three-dimensional gradient echo process.
In order to obtain a satisfactory image with the three-dimensional contrast-enhanced MRA, the following three points are important, i.e., (1) injection manner of the contrast agent, (2) image-taking timing, and (3) setting of optimum imaging conditions (especially a flip angle or an excitation angle).
Regarding the first condition (1), the contrast agent has to be injected in such a manner as allowing the contrast agent to stably maintain a high concentration in blood vessels to be imaged. For that purpose, the contrast agent is generally quickly injected using an automatic injector.
Regarding the second condition (2), to selectively take an image of only the artery distinguished from others, the image-taking timing has to be set such that the concentration of the contrast agent is kept high in the artery at the time of collecting data.
In particular, it is ideal that a central portion (low frequency region) of the k-space, on which an image contrast depends dominantly, is measured in match with the timing at which the concentration of the contrast agent is peaked. The image-taking timing is set corresponding to a method of collecting data by a pulse sequence. The technique for setting the timing is disclosed in Patent Document 1 given blow.
Regarding the third condition (3), for the purposes of minimizing signal attenuation due to shortening of T2 caused by the contrast agent and phase dispersion caused by blood flows, TE time from excitation to echo center) is set to a value as short as possible (not longer than 3 ms), and TR is set to a relatively short time (not longer than 10 ms) within the allowable range of S/N depending on the injection speed of the contrast agent. In other words, a method of changing TR is modified following the concentration of the contrast agent.
Patent Document 1: U.S. Pat. No. 5,553,619
Regarding the third condition (3), in the known three-dimensional contrast-enhanced MRA, because since the gradient echo sequence with short TR is used, an optimum flip angle has to be set depending on the concentration of the contrast agent in the vascular lumen. Usually, the optimum flip angle is set to the Ernst's angle at the estimated concentration of the contrast agent, which corresponds to the time when the concentration of the contrast agent is peaked.
However, the concentration of the injected contrast agent in the vascular lumen is changed at every moment with time such that the concentration increases exponentially until reaching the peak and then decreases exponentially after reaching the peak. Thus, there is a problem that the optimum flip angle is given just at the concentration peak, and high signals cannot be obtained from blood flows over the entirety of a measurement period.
In the three-dimensional contrast-enhanced MRA of the related art, the image-taking conditions are optimized, including optimization of the flip angle, to be matched with the peak of the concentration of the contrast agent in the vascular lumen within a target area (see the techniques disclosed in Patent Document 1).
Stated another way, because the concentration of the contrast agent in the vascular lumen, which has been quickly intravenously injected through the vein, is changed at every moment with time, measurement in a time zone where the concentration of the contrast agent is peaked means that the measurement is optimized in point of maximizing the intensity of an echo signal.
However, the measurement in time zones before and after the zone corresponding to the peak of the concentration of the contrast agent cannot be regarded as satisfactory from the viewpoint of optimum measurement.
With the techniques disclosed in Patent Document 1, since the timing of acquiring data regarding the central portion of the k-space is matched with the peak of the concentration of the contrast agent in the vascular lumen within the target area, the measurement of the central portion of the k-space, which predominantly contributes to the image contrast, is optimized and high signals can be obtained in the central portion. With the techniques disclosed in Patent Document 1, however, the measurement of peripheral portions of the k-space, which predominantly contributes to the image contour (sharpness), is away from the optimum state, and therefore satisfactory high signals cannot be obtained in the peripheral portions.